This invention relates to a method of generating a three-dimensional rotationally symmetric quadrupole electric field or an electric field of higher multipole moments inside an electrode structure forming the boundary of the field by application of a resultant electric potential .PHI..sub.q0 to the electrode structure.
Up to now, three-dimensional rotationally symmetric quadrupole fields were generated by an array of metallic electrodes with hyperbolic isopotential surfaces (U.S. Pat. No. 2,939,952 and U.S. Pat. No. 3,527,939). As an example in FIG. 1 the standard structure is shown, which consists of a ring electrode (1) of radius r and two end caps (2) of distance 2z.sub.0 .multidot.r.sub.0 and z.sub.0 are characteristic dimensions, which are related to the spacings of the hyperbolic surfaces from the center of the structure. The application of the three-dimensional rotationally symmetric quadrupole field to trap ions and charged particles and to study the properties of the trapped species and to generate mass spectra is well reported in the literature (Quadrupole Mass Spectrometry and Its Applications, P. H. Dawson, Ed., Elsevier, Amsterdam, 1976, and D. Price and J. F. J. Todd, Int. Mass Spectrom. Ion Processes, 60 (1984) 3).
For the generation of mass spectra chiefly four methods are described:
Mass analyzer method, disclosed in U.S. Pat. No. 2,939,952, PA1 The mass-selective storage method disclosed in U.S. Pat. No. 3,527,939, PA1 The mass-selective instability method disclosed in U.S. Pat. No. 4,540,884, PA1 Detection of image currents disclosed in U.S. Pat. No. 2,939,952, published in E. Fischer, Z. Phys., 156 (1969) 26, employing Fourier Transformation. PA1 The manufacturing of electrodes is complicated and costly. PA1 Due to the finite size of the electrodes, field imperfections are generated. PA1 Since gaps exist between ring and cup electrodes the resulting quadrupole field is easily influenced by charges accumulated on the surface of the electrodes. PA1 The detection of the image current signal generated by the ions is disturbed by other electric fields. PA1 The image current generated by the charged particles depends on their position in the trap, resulting in a noise signal.
The generation of a three-dimensional electric quadrupole field by hyperbolically shaped metallic electrodes generates several severe problems:
Finally, there is one further important disadvantage in generating a three-dimensional electric quadrupole field using hyperbolically curved electrodes: It is impossible to generate additional electric fields within the same interior region of the electrodes without any interference with the first electric field.
However, employing metallic electrodes with hyperbolic surfaces is not the only possibility of generating three-dimensional quadrupole fields, although up to now only electrode surfaces following the equipotential surfaces at the boundary of the electric field are commonly used because of prejudice.
Accordingly, it is an object of the invention to provide a method and the corresponding structures for generating a three-dimensional quadrupole electric field or an electric field of higher multiple moments which is much more exact, using no hyperbolically curved metallic electrodes and thus presenting the possibility of superimposing additional homogeneous electric fields without interference with the first electric field.